1. Field of the Invention
The present invention relates to an optical module to be utilized in an optical branching and inserting apparatus for branching a signal light from a trunk line toward a relay station and inserting the signal light transmitted from the relay station to the trunk line in an optical communication field, for example.
2. Description of the Related Art
In an optical communication using wavelength division multiplexing, an optical branching and inserting apparatus disclosed in JP-A-2000-183816 has been known as an apparatus to be used for branching a signal having a specific wavelength into a relay station and inserting the signal having a specific wavelength from the relay station.
As shown in FIG. 3, the optical branching and inserting apparatus has an optical branching device 3 for branching a wavelength multiplexing light input from a light transmission path 1 for input, and an optical coupling device 4 for coupling lights having respective wavelengths which are once branched and transmitting the lights to an output transmission path 2. The optical branching and inserting apparatus comprises a plurality of optical switches 5 corresponding to optical paths having respective wavelengths which serves to select to branch a light having each wavelength branched by the optical branching device 3 into a receiver 7 of a relay station 8 and to newly insert a signal transmitted by a transmitter 6 of the relay station 8 or to exactly transmit the light having each wavelength branched by the optical branching device 3 to the optical coupling device 4.
In such a branching and inserting apparatus, a filter module having the function of fixing a wavelength selecting filter or a lens onto an emitting optical path from an optical fiber and separating a single wavelength component from a multiple wavelength signal or the function of inserting the single wavelength component into the multiple wavelength signal is often used in the optical branching device 3 or the optical coupling device 4.
Such a filter module has a structure in which collimators including a lens and an optical fiber are provided opposite to each other with a wavelength selecting filter interposed therebetween as described in JP-T-10-511476 and JP-A-10-311905, for example.
In such a filter module, generally, a wavelength selecting filter, a lens and an optical fiber are inserted and fixed into a common cylindrical housing with an optical axis adjusted. Such a module is generally referred to as an Add/Drop Multiplexer (ADM).
Since the optical branching device 3 and the optical coupling device 4 in the optical branching and inserting apparatus of FIG. 3 are to carry out the same coupling or branching for a plurality of wavelengths, they have such a structure that a plurality of filter module units having different wavelength separating characteristics is used and the optical fibers on signal input/output ends thereof are sequentially connected by a method such as fusion. Such a module is generally referred to as “Mux/DeMux”. A light to be input to the optical branching device 3 or the optical coupling device 4 sequentially passes through a plurality of filter modules to be branched to have each wavelength or a light having each wavelength is sequentially coupled (for example, see JP-A-11-337765). In general, the single modules connected sequentially are attached to a single case.
In the optical branching and inserting apparatus using the filter module, if the number of channels to be used for an optical communication is increased, it is necessary to correspondingly increase the number of single filter modules to be used. For this reason, the price of a raw material component is equal to or more than a multiple of the price of the single filter module. Moreover, there is provided the step of fusing the optical fiber on the input/output end of the filter module. Therefore, the step is complicated and a cost is increased, and furthermore, a connecting loss is caused by a transverse offset during fusing connection. Furthermore, the single filter module has such a structure as to be fixed into the housing. Consequently, there is a problem in that an unnecessary volume other than functional parts is required and the volumes of necessary components are also increased with an increase in the number of the channels.
In order to eliminate these drawbacks, the inventors tried to reduce the price, size and loss of an optical module in a minimum volume without using unnecessary components by a structure in which an exterior member to be the housing of the filter module is eliminated and the components are fixed onto a single substrate, and a light is spatially propagated between the components.
However, it was found that the shift of an optical axis is generated on a light emitted from each component so that optical coupling cannot be easily carried out and an expected performance cannot be obtained in the case in which the element components in the module are to be actually separated and provided on the substrate.
The factor for the shift of the optical axis can include the following:
the end faces of an optical fiber and a refractive index profile type lens are set to be oblique end faces in order to reduce a reflection loss;
the optical axis is shifted when a light is transmitted through the substrate of a dielectric multilayer film filter to be a wavelength selecting filter;
fabrication can be carried out with precision in the external shape of each component which is equal to or less than precision in a processing required for optically coupling single mode fibers; and
fabrication can be carried out with precision in a processing of a substrate to be provided with these components which is equal to or less than precision required for optically coupling the single mode fibers.
The contents will be specifically described. For the optical coupling of the optical fibers, particularly, the single mode fibers, precision in alignment on a submicron level is required because a core diameter is 10 μm or less. In passive optical components such as a fiber pigtail and a lens, a component tolerance and a manufacturing tolerance exceed the same precision. Actually, the fabrication cannot be carried out with the same precision. Even if the fabrication can be individually carried out, moreover, there is a problem in that an emitted light is shifted from the optical axis in a collimator fabricated by a manufacturing method which is a current mainstream.
FIG. 4 shows a collimator fabricated by the manufacturing method which is the current mainstream, that is, in combination of a fiber pigtail 11 and a refractive index profile lens 12. In order to reduce a reflection loss, an angle of approximately 8 degrees is formed on each of the end faces of the pigtail 11 and the lens 12. Consequently, a position shift δ and an angle shift θ are generated on an emitted light as compared with the position of an incident light. In particular, the amount of the shift of the optical axis caused by the angle shift θ is increased if a coupling distance L is increased as shown in FIG. 5. In a collimator pair provided in a V groove on the same straight line, accordingly, the optical coupling is almost zero when a space is several mm or more.
In the case in which the V groove for fixing the collimator onto a substrate is fabricated by grinding, moreover, it is desirable that two V grooves provided with the collimator pair should be formed in parallel with each other at a request of a work. For the above reason, the collimator pair for implementing effective optical coupling cannot be fabricated on the V groove.
Moreover, an interference filter such as a wavelength selecting filter is usually fabricated by forming a film on a glass substrate 15 having a finite thickness as shown in FIG. 6 and has a thickness of approximately 1 mm to avoid a breakdown against a generated film pressure. The parallel positional shift amount δ of a light incident at an angle of incidence θ on a medium 2 having a thickness h and a refractive index n2 from a medium 1 having a refractive index n1 (=a difference between an optical path to be passed when the medium 2 is not present and an actual optical path) can be expressed in the following equation.
                    δ        =                  h          ⁢                                          ⁢          sin          ⁢                                          ⁢                      θ            [                          1              -                                                cos                  ⁢                                                                          ⁢                  θ                                                                                                                    (                                                                              n                            2                                                                                n                            1                                                                          )                                            2                                        -                                                                  sin                        2                                            ⁢                      θ                                                                                            ]                                              Equation        ⁢                                  ⁢        1            
FIG. 7 shows a relationship between the shift amount δ (μm) of the optical axis and the angle of incidence θ (Degree) when a light passes through a substrate having various thicknesses (0.5 to 1.5 mm) as shown in FIG. 6. As shown in FIG. 7, the shift of the optical axis is generated depending on the thickness of the substrate and the angle of incidence. Even if the optical coupling of the collimator pair is previously carried out before the interference filter is inserted, therefore, the optical path is shifted by the simple insertion of the filter so that a loss can be greatly increased or the coupling cannot be carried out.
Even if all the shifts are estimated to carry out a design, furthermore, a processing error and an assembly error of a component and a substrate are generated on each component. In addition, these errors have a level which clearly departs from necessary precision for the optical coupling, which is insignificant.
As described above, there is a problem in that the shift of the optical axis is actually great and sufficient optical coupling cannot be obtained if each component is simply arranged in parallel in each V groove for component fixation which is formed on the same substrate as in a conventional trial.